IMON protects before it reaches any email client as it intercepts it as it comes off the internet/network.

The reason why it is an important distinction is that any email scan arrangement along the lines of EMON might conceivable by circumvented by a flaw or vulnerability in the email client. Doing scans while it is just coming "off the wire" (such as via IMON) helps ensure reliability in this respect and can be used for other email clients. As an example, I have my Opera/M2 email protected via IMON.